Collections.reverseOrder() in Java with Examples
java.util.Collections.reverseOrder() method is a java.util.Collections class method.
// Returns a comparator that imposes the reverse of
// the natural ordering on a collection of objects
// that implement the Comparable interface.
// The natural ordering is the ordering imposed by
// the objects' own compareTo method
public static Comparator reverseOrder()
We can the comparator returned by Collections.reverseOrder() to sort a list in descending order.
Sample Code:
import java.util.ArrayList;
import java.util.Collections;
/**
* @author Abhinaw.Tripathi
*
*/
public class ReverseOrderApp {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
ArrayList<Integer> al = new ArrayList<Integer>();
al.add(30);
al.add(20);
al.add(10);
al.add(40);
al.add(50);
/* Collections.sort method is sorting the
elements of ArrayList in descending order. */
Collections.sort(al, Collections.reverseOrder());
// Let us print the sorted list
System.out.println("List after the use of Collection.reverseOrder()"+
" and Collections.sort() :\n" + al);
}
}
Output:
List after the use of Collection.reverseOrder() and Collections.sort() :
[50, 40, 30, 20, 10]
We can use this method with Arrays.sort() also.
Sample Code:
import java.util.*;
public class Collectionsorting
{
public static void main(String[] args)
{
// Create an array to be sorted in descending order.
Integer [] arr = {30, 20, 40, 10};
/* Collections.sort method is sorting the
elements of arr[] in descending order. */
Arrays.sort(arr, Collections.reverseOrder());
// Let us print the sorted array
System.out.println("Array after the use of Collection.reverseOrder()"+
" and Arrays.sort() :\n" + Arrays.toString(arr));
}
}
Output:
Array after the use of Collection.reverseOrder() and Arrays.sort() :
[40, 30, 20, 10]
public static Comparator reverseOrder(Comparator c)
Sample Code:
It returns a Comparator that imposes reverse order of a passed Comparator object. We can use this method to sort a list in reverse order of user defined Comparator. For example, in the below program, we have created a reverse of user defined comparator to sort students in descending order of roll numbers.
// Java program to demonstrate working of
// reverseOrder(Comparator c) to sort students in descending
// order of roll numbers when there is a user defined comparator
// to do reverse.
import java.util.*;
import java.lang.*;
import java.io.*;
// A class to represent a student.
class Student
{
int rollno;
String name, address;
// Constructor
public Student(int rollno, String name,
String address)
{
this.rollno = rollno;
this.name = name;
this.address = address;
}
// Used to print student details in main()
public String toString()
{
return this.rollno + " " + this.name +
" " + this.address;
}
}
class Sortbyroll implements Comparator<Student>
{
// Used for sorting in ascending order of
// roll number
public int compare(Student a, Student b)
{
return a.rollno - b.rollno;
}
}
// Driver class
class Main
{
public static void main (String[] args)
{
ArrayList<Student> ar = new ArrayList<Student>();
ar.add(new Student(111, "bbbb", "london"));
ar.add(new Student(131, "aaaa", "nyc"));
ar.add(new Student(121, "cccc", "jaipur"));
System.out.println("Unsorted");
for (int i=0; i<ar.size(); i++)
System.out.println(ar.get(i));
// Sorting a list of students in descending order of
// roll numbers using a Comparator that is reverse of
// Sortbyroll()
Comparator c = Collections.reverseOrder(new Sortbyroll());
Collections.sort(ar, c);
System.out.println("\nSorted by rollno");
for (int i=0; i<ar.size(); i++)
System.out.println(ar.get(i));
}
}
Output :
Unsorted
111 bbbb london
131 aaaa nyc
121 cccc jaipur
Sorted by rollno
131 aaaa nyc
121 cccc jaipur
111 bbbb london